The present invention relates generally to discharge lamps, in particular for vehicle illumination devices.
Discharge lamps of the above mentioned general type are known in the art. One of such discharge lamps is disclosed in the German patent document DE 43 19467 A1. This discharge lamp has a base and a burner connected with it. The burner has a discharge vessel with two electrodes which form a light arc during the operation of the discharge lamp. The electrodes are connected correspondingly with the electrical conductors. One electrode is connected with a supply conductor in the burner which is surrounded by a glass tube, while another electrode is connected with a return conductor which is located outside of the burner and extend along the burner to the base. The burner is mechanically connected with the base by the glass tube surrounding the supply conductor, and therefore the glass tube must have a sufficiently large cross-section, to reliably hold the burner. Also, constructions of discharge lamps are known in which the mounting of the burner is performed through a glass tube which surrounds it. Here however the inner glass tube must have a sufficiently large cross-section, so that the discharge vessel can be formed from it. An air-filled space remains between the surface of the supply conductor and the inner surface of the glass tube.
In order to provide a compact construction of the discharge lamp including the base, it is desirable to guide the returned conductor as close as possible on the glass tube to the base. For actuating the discharge lamp, it must be ignited, and for this purpose high frequency high voltage is applied through the conductors to the electrodes and can reach several tens kilovolt. It must be guaranteed that it does not lead to undesired partial discharges in the glass tube or ark-overs outwardly of the discharge vessel, which occurs in particular during an ignition of the discharge lamp with a high frequency high voltage of several megahertz. These partial discharges outside the discharge vessel represent on one side a danger and lead on the other side to braking of the lamp ignition since it takes away the required ignition energy. It has been shown that a strong electrical field is formed in particular between the supply conductor and the return conductor near the base or in the region of the base under the influence of the high voltage. It is caused by the available air-filled space between the surface of the supply conductor and the inner surface of the glass tube, which space has a low electrical insulation action. Especially pronounced are these disturbing partial discharges during the operation of the discharge lamp with reduced outer air pressure, which as a rule occurs at great altitudes.